The offshore production of oil and gas requires the use of substantially vertical, fluid-carrying pipes known as "flowlines" to convey fluids from a subsea site to the water surface. Frequently, a number of flowlines are contained in a vertical tension member known as a "riser". The upper end of the riser is connected to a floating vessel or buoy which is subject to angular and lateral movement due to wind, waves, and ocean currents. In some cases, a floating vessel dedicated to the collection of liquid petroleum is anchored in such a way that it is permitted to swing through a complete circle about the top of the riser without restraint. In any event, to compensate for this movement about the stationary subsea riser base, articulated joints are often installed in the riser. Swivels or articulated joints may also be installed in the flowlines to allow the riser to move without damaging the flowlines. For example, one application of swivels in flowlines is disclosed in U.S. Pat. No. 4,318,423 to DeGraaf, issued in 1982.
Swivels in risers and other fluid-carrying systems must be adequately sealed to prevent leakage of the fluids contained therein into the ambient surroundings. Such fluids will hereinafter be referred to "production fluids". Typically, plastic or elastomeric fluid seals are utilized between the moving parts of a swivel to prevent leakage of the production fluid. Although it is desirable that the fluid seals in a fluid carrying system be absolutely reliable over the design life of the system, experience has shown that the seals in such a swivel need be occasionally replaced.
The demands on existing fluid seal technology have increased as the quest for crude oil and gas extends into newly discovered reservoirs in deeper waters. Such reservoirs are often located at depths far below the earth's surface, and fluids from such reservoirs are often produced at high temperatures and pressures. For example, deep gas wells may produce fluids at temperatures higher than 450.degree. F. and pressures exceeding 10,000 lbs. per square inch. In addition, the fluids produced from such reservoirs are frequently sour and contain high concentrations of hydrogen sulfide or other acid gases.
Although fluid seals have been developed which have good resistance to deterioration induced by the action of sour hydrocarbons, these "production seals" tend to deteriorate when subjected to high temperatures and excessive pressures. For example, certain product seal elastomers resistant to chemical deterioration soften as the temperature of the product seal is increased. This softening reduces the tensile strength of the product seal and reduces its ability to resist damage due to excessive pressure. Conversely, other product seal elastomers become brittle at high temperatures. This embrittlement tends to cause the product seal to crack. Furthermore, such embrittlement reduces resiliency of the product seal which thereafter lessens its sealing effectiveness.
Replacement of product seals heretofore has often required complete diassembly of a swivel. Such a disassembly may result in damage to the sealing surfaces since the parts of the swivel having sealing surfaces machined therein are usually quite large and heavy. Consequently, these parts are very bulky and difficult to maneuver; damage to sealing surfaces may result during disassembly and assembly. Furthermore, the included bearings in such a swivel must be removed before machining of the sealing surfaces may be attempted.
Thus, a need clearly exists for a flowline swivel having product seals which are easily replaceable, having sealing surfaces which are removable without also disturbing the swivel bearing, and having a bearing which is protected from the intrusion of production fluids during operation.